Electronic reproduction on output devices of continuous tone images, together with graphical information such as line art and characters usually involves a step for the generation of a bitmap. If the output device is a binary system, capable to deposit at every addressable location a fixed amount of ink or nothing, a binary bitmap must be generated. Such binary bitmap can be seen as a rectangular array of elements, wherein each element corresponds to one addressable location or recorder element. Each element in the bitmap must be able to represent that ink must be deposit or not, and can therefore be represented by one bit, having e.g. a value 0 for ink, and 1 for no ink. If the resolution of the output device is 400 dpi (dots per inch) or about 16 dots per mm, and the maximal paper size is an A3 form, measuring 420 mm by 297 mm, such as for the AGFA XC305 and AGFA XC315 system, marketed by Agfa-Gevaert N.V. in Mortsel, Belgium, a bitmap covering such a form requires about 29 megabits of memory, or over 3.6 megabyte. Several output devices, such as the XC305 and XC315 systems, have multilevel or contone capabilities. This means that every recorder element can be rendered with intermediate density levels, between no ink deposit and full ink deposit. As such, the above mentioned systems accept eight bits per recorder element, such that a bitmap covers 29 megabytes of memory. Moreover, these output devices are capable to print with different coloured inks, and can print four inks (cyan, magenta, yellow and black) on top of each other on the same page, to reproduce a continuous colour image. The four corresponding bitmaps require nearly 120 megabytes of memory. Most of these output devices require that the data are delivered in real time to the printing engine. This means that the bitmaps must be pre-computed and stored in 120 megabytes of memory, or that the four bitmaps must be generated in real time, keeping up with the speed of the printing process of the output device. The first solution requires a huge amount of expensive memory. The second solution is not feasible for complex page layouts. For most output devices it is also not appropriate to stop the printing process between the printing of two colour components on top of each other. This is due to the fact that the paper on which the ink must be deposit is usually preconditioned to regulate its temperature and humidity. If the printing process is stopped for some tens of seconds, e.g. to compute the bitmap for the next colour component, the paper can take up additional humidity and swell, causing mis-registration of the next colour component with respect to the previous colour component. To avoid these problems, the cost of the mechanics of the printer would increase. Even if that problem were solved, a considerable amount of memory is necessary to store one full bitmap. As in photographic recorders, the bitmap could be computed in bands and every band could be printed once it is ready. But this causes tremendous problems for output devices working on the basis of an electrographic process. For such a process, the operational parameters for e.g. the toner brush are optimised such, that the system works best for a specific speed of the acceptor material, i.e. the photosensitive drum and the paper. This means that for most electrographic systems the bitmaps must be "ready to print" before the printing starts, or must be in such format that the bitmaps can be generated "on the fly", i.e. in real time with respect to the printing engine. A considerable amount of memory to store the bitmaps is thus necessary.
One system that reduces the size of the required memory with a factor of two, is disclosed in the European patent application 93202522.4 with priority date on Aug. 27th 1993 and titled: "High quality multilevel halftoning for colour images with reduced memory requirements". By transforming the contone levels to bitmap elements represented by 4 bits, the memory requirements can be reduced by a factor of two, without noticeable quality loss.
Another solution is incorporated in the Agfa Multistar 400 and 600 system. This system multiplexes different RIP (Raster Image Processor) outputs for one single recorder, for imaging the rasterized images on a photographic medium. The bitmaps for each colour component from the different RIP's are acquired, compressed by a lossless compression and temporarily stored on disk, along with a job list. The compression must be lossless, such that the bitmap information can be reconstructed exactly as it was generated, otherwise clearly visible artifacts would become apparent. By decompression of the data on disk, each bitmap is generated in bands, and each band is sent to the recorder. The drawback of a lossless compression is that the compression ratio ranges between a factor 1 and 8, but cannot be predicted accurately. If for example very complex halftone dot distributions are created by a high quality screening process, such as the Agfa CristalRaster (CristalRaster is a trade mark of Agfa-Gevaert N.V. in Mortsel, Belgium), it is even possible that the compressed bitmap is larger than the original bitmap. Moreover, the Agfa Multistar 400 and 600 cannot deliver the rasterized image data in real time to the recorder, which poses no problem for the photographic recorders, but is unacceptable for e.g. electrographic printers.